Body mass index (BMI)

background

Body mass index (BMI) is a tool for indicating weight status in adults. It is a measure of weight for height. For adults over 20 years old, BMI falls into one of these categories: Below 18.5 (underweight), 18.5-24.9 (normal), 25.0-29.9 (Overweight) and 30.0 and above (obese).
BMI correlates with body fat; BMI does not measure body fat. The relation between fatness and BMI differs with age and gender. For example, women are more likely to have a higher percent of body fat than men with the same BMI. On average, older people may have more body fat than younger adults with the same BMI. A BMI calculator may overestimate body fat in athletes and others who have a muscular build, and it may underestimate body fat in older persons and others who have lost muscle mass because it does not consider lean body mass.
BMI alone is not diagnostic. BMI is just one of many factors related to developing a chronic disease (such as heart disease, cancer, or diabetes). Other factors that may be important to look at when assessing a risk for chronic disease include diet, physical activity, waist circumference, blood pressure, blood sugar level, cholesterol level and family history of disease. As a person's BMI increases the risk for many diseases increases as well.
BMI is different for adults than it is for children. For children (2-20 years-old), a BMI-for-age is used to assess underweight, overweight, and risk for overweight. This is why BMI for children is gender and age specific, and is plotted on gender specific growth charts. Typically, BMI-for-age breaks down into categories: <5th percentile (underweight, 5th percentile to <85th percentile (normal), 85th percentile to <95th percentile (at risk of overweight), and ≥85th percentile (overweight).
Overweight and obese health risks: Being overweight or obese increases the risk of health conditions, such as stroke, cardiovascular disease, hypertension (high blood pressure), type 2 diabetes, dyslipidemia (high cholesterol or triglycerides), osteoarthritis, respiratory problems, and cancer. Overweight and obesity result from an energy imbalance. This involves eating too many calories and not getting enough physical activity. Body weight is the result of genes, metabolism, behavior, environment, culture, and socioeconomic status. However, behavior and environment play a large role in causing obesity, and these are the places to look to for prevention and treatment actions.
Underweight health risks: A low body mass can decrease the body's immunity, causing disappearance of periods (women), bone loss, malnutrition or other conditions.
Therapy recommendations: Aim for a healthy weight. Most experts believe that people who need to lose weight should do so gradually, at a rate of one-half to two pounds per week. Be active. The safest and most effective way to lose weight is to reduce calories and increase physical activity. Eat well. Select sensible portion sizes and follow the Dietary Guidelines for Americans.
The best way to add weight is to increase an individual's intake of complex carbohydrates, particularly whole grain ones. Foods such as whole wheat bread, muffins, pasta, crackers, bagels, legumes and fruits are good choices. In order to gain weight, an individual will have to eat more calories, and include regular exercise and strength training into her lifestyle in order to prevent gaining too much weight as fat. One should aim at gaining ½lb a week.
Overall experts believe that healthy diets include 6-8 glasses of distilled water a day and lots of raw fruits and vegetables (green leafy vegetables especially). Some may find it helpful to eat frequent small meals throughout the day to keep energy and metabolism increased. Coffee, alcohol, soda pop, processed foods; white sugar, white flour, red meat and animal fats should be avoided.

Related Terms

Adiposity, body surface area, BMI, BMI calculator, head circumference-for-age, lean body mass, obese, overweight, stature-for-age, underweight, waist circumference, weight-for-age and weight-for-length percentiles.

charts and calculators

For adults, the accepted formula for calculating adult (20 years and older) BMI is:
English: BMI=703 x [Weight in pounds/(Height in inches)2]
Metric: BMI= [Weight in kilograms/(Height in meters)2]
Note: 2.2 lb=1kg
Body Surface Area = 0.20247 x Height(m)0.725 x Weight(kg)0.425
The formula for adult BMI can be used with children, although the results are interpreted differently. BMI charts showing percentiles are used with children 2-20 years of age. Related growth charts include head circumference-for-age (infants 0-36 months), weight-for-length percentiles (infants 0-36 months), stature-for-age (2-20 years) and weight-for-age (2-20 years).
Although there is general consensus that BMI for adults is associated with increased disease risk, there are differences in BMI for men and women, and older and younger adults. The graphs below show that older children's (boys) BMI increases up to age 20, (and keeps on increasing after that). This data came from the standard CDC growth charts. There is a gap between the children and adult data, because the data sources are different. The adult data (from the NHANES III survey) comes from data collected between 1988 and 1994, whereas the CDC body-mass-index-for-age charts are derived from a mix of data from new and older sources. A similar chart is provided for the women. The statement that over 50% of Americans are overweight relies on these graphs: the 50th percentile of men have a BMI of 25 or above, which is considered overweight according to most CDC charts. Similarly, 50% of American women over 30 also have a BMI that is considered overweight and at risk.

cdc charts